Journal Article Heat as a Tracer for Examining Depth-Decaying Permeability in Gravel Deposits

Sakata, Yoshitaka

53 ( S1 )  , pp.21 - 32 , 2015-04 , Wiley-Blackwell
Depth dependence of permeability can appear in any geologic setting; however, vertical trends in alluvial gravel deposits are poorly understood because of the high variability of hydraulic conductivity K in monotonic sequences. This paper examines the sensitivity of depth-decaying permeability through heat transport simulation around a river's losing reach in the Toyohira River alluvial fan, Japan. Observed variations in groundwater temperature indicate that heat fluxes are dominant in the shallow zone, despite a vertical hydraulic gradient. In eight cases with different conditions (presence or absence of exponential decay trend, large or small variogram range, and cell isotropy or anisotropy) 1000 K realizations are stochastically generated throughout a cross-sectional model. The groundwater flow and heat transport are transiently calculated, and the averaged root mean square error RMSE?</mml:mover> is used for sensitivity comparison. The variance of RMSE?</mml:mover> shows that small RMSE<mml:mo stretchy="true">?</mml:mover> realizations are effectively reproduced with vertical trend assumed. Plausible realizations of <mml:mover>RMSE<mml:mo stretchy="true">?</mml:mover> below a given threshold were obtained only when a vertical trend was assumed. The most plausible realization almost completely matched the observations. However, the number of plausible realizations per case was 10 and the median <mml:mover>RMSE<mml:mo stretchy="true">?</mml:mover> were insensitive to all the conditions. Statistical testing suggested that these plausible realizations may be statistically significant, aiding in generating a connected K zone for high heat flows. The cell anisotropy condition had the smallest effect on the simulation. Thus, effective modeling of the vertical trend contributes to heat transport; however, the model's efficiency is low without detailed information about the sedimentary structure.

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